The Universal Serial Bus (USB) is a standardized interface for data communications between electronic devices. Electronic devices which incorporate the USB may communicate with each other utilizing standard connectors and interface protocols.
The USB as originally designed is based on a master-slave protocol wherein a host system (master) may connect to one or more peripheral devices (slaves) in a tiered star topology. The host system may control several peripheral devices through a series of hubs. The host system determines how connections and communications are made to the peripheral devices, and therefore the intelligence resides primarily in the host system.
USB uses directional connectivity wherein one type of connection (mating pair of plug and receptacle) is used to connect to an upstream host device and a different type of connection is used to connect to a downstream peripheral device. A host, according to the USB specification, may include a Series A receptacle that only connects to a Series A plug, while a peripheral device may include a Series B receptacle that only connects to a Series B plug. The connection between such host and peripheral device is thus made by a USB cable with a Series A plug at one end and a Series B plug at the other. Other peripheral devices, such as a memory stick, may be equipped with a Series A plug in which case direct connection between the peripheral device and the host can be made without a cable. USB also envisions that the host acts as the source of power.
This directional connectivity as well as the power distribution requirement as defined by the USB specification place certain limitations on the interconnectivity of electronic systems using the Series A/Series B connectors. For example, an electronic device that may be able to act as a host in one mode of operation and as a peripheral device in another mode of operation cannot, by definition, use the same Series A port in both modes of operation. While USB has defined a separate interface, called On-The-Go (OTG), for dual role devices (i.e., devices that can be configured to operate either as a host device or a peripheral device), the USB OTG specification requires different connectors (Mini-A, Mini-B and Mini-A/B) that do not mate with the Series A and Series B connectors. Therefore, dual role electronic devices that need a Series A port must also provide an additional connection port to enable them to connect to a host when operating as a peripheral device.